Field of the Invention
The present invention relates to a toner for electrophotography. Specifically, the present invention relates to a resin composition used in the toner for electrophotography which is used in printers or coelectrophotography machines utilizing heat-roll fixing.
Recently, various improvements or modifications on coelectrophotography machines or printers utilizing electrophotography have been desired as these machines became more popular. For example, it has been desired to achieve a low energy consumption in order to find their way into homes. It is also desirable to make the machines run at high speeds with a view to promoting the spread of the machines into so called gray areas positioned at a border line between printers and copiers. It is also necessary to simplify the structure of the machine in order to reduce the cost of the machines.
Moreover, it has become popular to use the machines having functions of printing on both sides of the printed sheet and delivering the printed sheet automatically.
Of the above-described requirements, particularly desired is a toner for electrophotography (hereafter, referred to simply as "toner") having a lower fixing temperature, a good anti-offset quality and fixing strength so that spots are not generated in a printed sheet in printing both sides of the printed sheet.
The toner used widely from the past has the following composition.
Binder resin: 80-90 parts by weight PA1 Pigment: 5-15 parts by weight PA1 Charge Controlling Agent: 0.5-5 parts by weight PA1 Additives (for example, Cleaning Agent, Fluidizer): 0.1-1 parts by weight PA1 column temperature: 40.degree. C. PA1 solvent: tetrahydrofuran PA1 current speed: 1 mm/min PA1 sample concentration: 0.2% PA1 sample amount: 100 .mu.l PA1 column: KF-80M x 2 (marketed by Shodex Co.) PA1 KF-802.5 (marketed by Shodex Co.)
It is clear from the above composition that most components comprising the toner are binder resins. Properties of the toner are influenced by the properties of the binder resin. Therefore, it is understood easily that the properties of the toner can be controlled by controlling the properties of the binder resin.
In order to give a good low temperature fixing property to the toner, it is preferable to use the binder resin having a lower molecular weight. By using a resin having a lower molecular weight as a binder resin, the fixing property at low temperatures can be improved; however, an offset phenomena can occur easily, and toner particles are easily coagulated with each other. Therefore a shelf life of the toner becomes worse.
Another means to solve the problems, for example, is to add a material having a low melting point. However, the same problems as above were encountered.
A Japanese Patent Application First Publication H4-361271 discloses a means of using thermoplastic polyesterurethane resin and vinyl copolymer, as one of the means to solve the problems. Thermoplastic polyesterurethane resin has urethane bonding at the end thereof, and the urethane bonding includes nitrogen atoms. Therefore a polarity of the toner comprising thermoplastic polyesterurethane resin becomes easily positive. When thermoplastic polyesterurethane resin is added to a negatively charged toner, there are cases when the negative charge of the toner becomes weak when the toner is used for a long time. Consequently, smudging is easily generated in a printed paper. In contrast, when thermoplastic polyesterurethane resin is added to the positively charged toner, there are cases when the positive charge property becomes stronger, therefore the image density decreases.
A Japanese Patent Application First Publication H6-289644 discloses a means of using a vegetable wax to provide a toner having a good fixing property even at lower temperatures. In general, the vegetable wax comprised wax ester, free fatty acid, and a minor alcohol component. The minor alcohol component is oxidated in a heating step of preparing the toner, therefore activated polar groups are generated. When the activated polar groups exist, frictional electrification of the toner used over a long time becomes unstable. Therefore an image density becomes lower. That is, the toner comprised of vegetable wax has a problem in image properties.